1. Field of Invention
This invention relates to patterned photoresist structures including features having high aspect ratios. This invention further relates to methods of forming the patterned photoresist structures.
2. Description of Related Art
Photolithography is widely used to make very accurate microscopic patterns in a material, such as photoresist. Traditionally, photoresist is coated onto a substrate, and light or other radiation passing through a patterned mask transfers the pattern into the resist layer. After development, the pattern exists in the photoresist and can be utilized. Commercial photoresists are complex blends of polymeric and other organic and inorganic materials. The two broad classifications of photoresists are negative and positive resists that produce negative and positive images, respectively. In a negative resists, regions that are exposed to light are polymerized and, consequently, more insoluble to the developer. Thus the regions that are not exposed to light are preferentially removed during development. Positive resists have different chemistries from negative resists. When regions of positive resists are exposed to light, they are changed to have a higher degree of solubility and are preferentially removed during development. The selection of a negative or positive photoresist would depend on the full details of the particular application; namely, details such as resolution, exposure equipment, chemical selectivity, film thickness and chemical requirements. Photoresists are patterned to form features. In many microelectronics applications, photoresists are removed from the underlying substrate after the substrate has been etched, transferring the complement of the pattern from the photoresist to the substrate. Wet stripping solutions, dry etching techniques and high-temperature ashing techniques may be used to remove the photoresists from the substrates.
In addition to their temporary use in microelectronics applications, photoresists have been used both as temporary sacrificial layers and as permanent structural layers in micromechanical devices, as described in "Photosensitive Polyimide: Lithography in the Thick-Film Regime," S. G. Hagen, R. E. Hopla, L. J. Peterson, D. W. Racicot, A. J. Roza, A. Schaffner and W. D. Weber, Proceedings of the Eleventh International Conference on Photopolymers, Society of Plastics Engineers, Inc., Oct. 6-8, 1997, pp. 422-437; which is incorporated herein by reference in its entirety.
One such known application of photoresist layers has been in ink jet printers. Ink jet print heads have a structure including a base plate and a cover plate. The ink jet print head can also include an intermediate layer disposed between the top plate and the bottom plate. The intermediate layer and the base and/or cover plates have structures that form ink channels and nozzles for flowing and discharging the ink onto a recording medium. Heating elements such as microresistors or piezo elements are provided in or on the base plate in alignment with the ink nozzles to cause ink droplets to be discharged out of the nozzles and onto the recording medium.
Photoresist layers have been used in a sacrificial mode to form the intermediate layer, as described in U.S. Pat. No. 5,738,799 to Hawkins et al. The photoresist layer represents the configuration of ink passages or capillary channels that are formed in the finished print head. The photoresist layer is removed during formation of the print head to define the ink passages. U.S. Pat. No. 5,290,667 to Shiba et al. describes a method for producing an ink jet print head that includes the use of positive photoresists to form ink paths in a layer formed on a substrate.
Photoresist layers have also been used as permanent structural layers in ink jet print heads. For example, U.S. Pat. No. 5,582,678 to Komuro describes using both negative photoresists and positive photoresists to form an intermediate layer in an ink jet recording head. The negative photoresist is patterned to form features having the configuration of ink pathways. The positive photoresist is filled into the ink pathways and then removed such that the negative photoresist forms a permanent structural layer including the ink pathways.
U.S. Pat. No. 5,557,308 to Chandrasekaran describes an ink jet print head including a negative photoresist layer disposed between a top plate and a bottom plate. The negative photoresist layer forms a permanent structural layer defining the ink channels in the ink jet print head.
It is also known to form the photoresist in multiple separate layers in ink jet print heads. For example, U.S. Pat. No. 5,375,326 to Usui et al. describes a method of manufacturing an ink jet print head in which a plurality of negative photoresist layers are successively applied on each other and patterned to form flow paths. U.S. Pat. No. 5,686,224 to O'Neill describes an ink jet print head that includes ink channel structures formed by patterning multiple coatings of a positive photoresist applied on a substrate. The channel structures include ink channels, heater pits and an ink manifold, each having different depths. Heating elements are located in the ink channels and heater pits are disposed over the heating elements. A cover plate covers the channel structures and includes an ink inlet for each ink manifold.
When the photoresist is used as a permanent structural layer, the openings, or features, can have various shapes and sizes. For example, the features can be relatively narrow and long, such as in lines or trenches. The sidewalls defining the features can be substantially vertical or can be tapered. Another type of opening or feature that can be formed in photoresist layers is an island. Islands are discrete upstanding structures formed on substrates. Islands have generally elongated shapes as disclosed in the incorporated Hagen reference. The opening or feature could also be a hole within an otherwise continuous area.
The features formed in photoresists can be characterized by their aspect ratio. The aspect ratio depends on both the height and width of a feature. For a typical feature, however, there will also be a certain amount of taper of the sidewalls. FIGS. 1 and 2 show two different opening configurations that have aspect ratios defined by respectively different relationships. FIG. 1 shows a photoresist layer 10 having a surface 12, and an opening 14 formed in the surface 12 having a height h and a width w. The height h can be less than or equal to the thickness of the photoresist layer 10. As shown, the side walls 16 defining the opening 14 are perpendicular to the surface 12. For this opening configuration, the aspect ratio A.R. can be defined as the ratio of the height h to the width w of the opening 14; that is, A.R.=h/w.
FIG. 2 shows a photoresist layer 20 formed on a substrate 22. A mask 24 used to pattern the photoresist layer 20 is shown positioned above the photoresist layer 20. The mask 24 includes openings 26 having a width b, and separated from each other by mask portions 25 having a width a. The photoresist layer 20 has a thickness h and includes an upper surface 28, a lower surface 30, and an opening 32 extending vertically between the upper surface 28 and the lower surface 30 and aligned with the opening b in the mask 24. The opening 32 is defined by side walls 34 which inwardly taper, such that the width of the opening 32 varies from a maximum width b' at the upper surface 28 to a minimum width b" at the lower surface 30. The photoresist layer 20 has a width a' at the upper surface 28 and a width a" at the lower surface 30. For the opening 32 having such tapered side walls 34, the average aspect ratio A.R. of the opening 32 can be defined as A.R.=2h/(b'+b"). Likewise, the average aspect ratio of the wall between the openings can be defined as A.R.=2h/(a'+a").
In addition, it is common for the aspect ratio to be described as a pair of numbers h:w (height to width of a feature).
In known methods and apparatuses, it is typical to be able to pattern a photoresist to aspect ratios of 1:1.